84 lines
1.9 KiB
C
84 lines
1.9 KiB
C
#include "tommath_private.h"
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#ifdef MP_REDUCE_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/* reduces x mod m, assumes 0 < x < m**2, mu is
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* precomputed via mp_reduce_setup.
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* From HAC pp.604 Algorithm 14.42
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*/
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mp_err mp_reduce(mp_int *x, const mp_int *m, const mp_int *mu)
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{
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mp_int q;
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mp_err err;
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int um = m->used;
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/* q = x */
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if ((err = mp_init_copy(&q, x)) != MP_OKAY) {
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return err;
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}
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/* q1 = x / b**(k-1) */
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mp_rshd(&q, um - 1);
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/* according to HAC this optimization is ok */
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if ((mp_digit)um > ((mp_digit)1 << (MP_DIGIT_BIT - 1))) {
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if ((err = mp_mul(&q, mu, &q)) != MP_OKAY) {
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goto LBL_ERR;
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}
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} else if (MP_HAS(S_MP_MUL_HIGH)) {
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if ((err = s_mp_mul_high(&q, mu, &q, um)) != MP_OKAY) {
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goto LBL_ERR;
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}
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} else if (MP_HAS(S_MP_MUL_HIGH_COMBA)) {
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if ((err = s_mp_mul_high_comba(&q, mu, &q, um)) != MP_OKAY) {
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goto LBL_ERR;
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}
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} else {
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err = MP_VAL;
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goto LBL_ERR;
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}
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/* q3 = q2 / b**(k+1) */
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mp_rshd(&q, um + 1);
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/* x = x mod b**(k+1), quick (no division) */
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if ((err = mp_mod_2d(x, MP_DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
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goto LBL_ERR;
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}
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/* q = q * m mod b**(k+1), quick (no division) */
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if ((err = s_mp_mul(&q, m, &q, um + 1)) != MP_OKAY) {
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goto LBL_ERR;
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}
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/* x = x - q */
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if ((err = mp_sub(x, &q, x)) != MP_OKAY) {
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goto LBL_ERR;
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}
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/* If x < 0, add b**(k+1) to it */
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if (mp_cmp_d(x, 0uL) == MP_LT) {
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mp_set(&q, 1uL);
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if ((err = mp_lshd(&q, um + 1)) != MP_OKAY) {
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goto LBL_ERR;
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}
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if ((err = mp_add(x, &q, x)) != MP_OKAY) {
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goto LBL_ERR;
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}
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}
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/* Back off if it's too big */
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while (mp_cmp(x, m) != MP_LT) {
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if ((err = s_mp_sub(x, m, x)) != MP_OKAY) {
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goto LBL_ERR;
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}
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}
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LBL_ERR:
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mp_clear(&q);
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return err;
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}
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#endif
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